1. Field of the Invention
The present invention relates to a polyamide fiber, and particularly to a high-Young's modulus polyamide fiber having an excellent durability under high-temperature conditions.
2. Description of the Related Arts
Polyamides have such chemical and mechanical properties as a high strength, an abrasion resistance, a fatigue resistance, and a good dyeability so that they can be used as injection molding materials for housings or the like for appliances and computers and as industrial materials and materials for leisure goods, for example, for threads, clothing including textile fabrics, tire cords, fishing nets, canvas, filter fabric for papermaking, and tennis-racket strings, or can be used for food packaging films or for sheets for a variety of containers. Particularly, since polyamides obtained from xylylenediamine and aliphatic dicarboxylic acids and containing amide linkage repeating units, which are called polyamide MXD6, have a high Young's modulus and a low water absorption and are excellent in hydrolytic resistance in comparison with polyamide 6 with polyamide 66 and the like, they are particularly useful as the foregoing industrial materials, for example, for door mirror stays, tire cords, and filter fabric for papermaking. However, polyamides have defects that the mechanical strength is lowered in an atmosphere having a high temperature, particularly in the presence of oxygen, where lamp reflectors, tire cords, filter fabric for papermaking, and the like are actually used, and that the coloring is extremely facilitated in such an atmosphere, and polyamide MXD6 cannot be free from these defects also.
To protect polyamide products from deterioration due to thermal oxidation, conventionally, a variety of methods wherein copper compounds are mainly used has been suggested. Specifically, a method wherein a copper salt is added is described in British Patent No. 652,947. Nevertheless, the method is accompanied by a defect that the polyamide is colored reddish brown or a deposit insoluble in the polyamide separates when the polyamide containing such a copper salt is melted and molded. To overcome the defect, there are suggested, for example, a method disclosed in British Patent No. 839,067 wherein a copper compound is used together with an organic or inorganic halide with a method wherein a copper compound is used in combination with a phenolic age-resister or an aromatic amine age-resister with and a method disclosed in Japanese Patent Publication No. 22720/1963 wherein a copper compound is used in combination with a sulfur compound, such as 2-mercaptobenzimidazole. However, when polyamides, such as the above polyamide MXD6, are used, even these stabilizers have such problems that the coloring is facilitated extremely and the ability for heat stabilization is not exhibited satisfactorily. Thus, satisfactory stabilizers that solve all the above problems have not yet been found. Particularly in the field of industrial materials for tire cords, filter fabric for papermaking, and the like, the development of materials excellent in heat aging characteristics without impairing such advantages as a high strength, a high Young's modulus, and an excellent hydrolysis resistance is desired.